Time-lag cartridge fuse for D-C circuits

ABSTRACT

A time-lag cartridge fuse for d-c circuits has three different interrupting mechanisms, one for interrupting small overload currents, one for interrupting major fault currents, and one for interrupting intermediate overload currents. Small overload currents are interrupted by a first air break and a fusible shunt wire across said first air break. Major fault currents are interrupted by series breaks formed in a pulverulent arcquenching filler without resorting to fusible break-shunting means. Intermediate overload currents are interrupted by a second air break spaced from said first air break, and a fusible shunt wire across said second air break.

United States Patent Knapp, Jr.

[ 1 Jan. 28, 1975 [75] Inventor: Edward .1. Knapp. .1r., Newbury,

Mass.

[73] Assignec: The Chase-Shawmut Company,

Newburyport, Mass.

[221 Filed: May 8, 1974 [21] Appl. No.: 467,950

[52] U.S. Cl. 337/164, 337/163 [51] Int. Cl. H01h 85/12 [58] Field of Search 337/164, 163, 165, 166, 337/179, 181, 184

[56] References Cited UNITED STATES PATENTS 3,483,501 12/1969 Kozacka 337/166 3.504.322 3/1970 Belchcr et al 337/164 3,735,312 5/1973 Nagel 337/164 Primary ExaminerHarold Broome Attorney, Agent, or FirmErwin Salzer [57] ABSTRACT A time-lag cartridge fuse for de circuits has three different interrupting mechanisms, one for interrupting small overload currents. one for interrupting major fault currents, and one for interrupting intermediate overload currents. Small overload currents are interrupted by a first air break and a fusible shunt wire across said first air break. Major fault currents are interrupted by series breaks formed in a pulverulent arcquenching filler without resorting to fusible breakshunting means. Intermediate overload currents are interrupted by a second air break spaced from said first air break, and a fusible shunt wire across said second air break.

8 Claims, 5 Drawing Figures Patented Jan. 28; 1975 f, I v u t t ....(gggggggg v TIME-LAG CARTRIDGE FUSE FOR D C CIRCUITS BACKGROUND OF THE INVENTION This invention relates to an improvement of a family H. Taylor, June I5, 1943 for FUSIBLE ELECTRIC PROTECTIVE DEVICE: 3,483,50l to Frederick J. Kozacka, Dec. 9, I969 for ELECTRIC CARTRIDGE FUSE; 3,721,935 to Frederick J. Kozacka, Mar. 20, I973 for HIGH CURRENT-CARRYING-CAPACITY DUAL ELEMENT FUSE, and other patents. Fuses disclosed in these patents have gained wide acceptance in the trade because of their outstanding performance in many respects. These fuses are conceived as a-c interrupting devices and were, heretofore, generally used in connection with a-c circuits. To some extent fuses of the kind under consideration may also be used for interrupting d-c circuits, but their usefulness in connection with d-c circuits has, heretofore, been relatively limited. To be more specific, they could only be used in connection with d-c circuits having a relatively small circuit voltage and a relatively small time constant.

It is the prime object of this invention to adapt fuses of the kind under consideration for application in d-c circuits having onerous interrupting conditions, e.g., circuit voltages in the order of 700 volt, and having rel atively large time constants.

SUMMARY OF THE INVENTION Electric cartridge time-lag fuses embodying this invention include a tubular casing of electric insulating material and a pair of terminal elements closing the ends of the casing. A pair of spring-biased relatively movable contacts is arranged inside said casing adjacent the center region thereof. The casing contains a pulverulent arc-quenching filler. A sub-housing for said pair of contacts is arranged inside said casing, separating said pair of contacts from said pulverulent arcquenching filler. A first fusible element'in ribbon-form having a plurality of serially related points of reduced cross-sectional area and having a predetermined minimum cross-sectional area interconnects conductivelyone of said pair of terminal elements and one of said pair of contacts. A second fusible element in ribbon form having a plurality of serially related points of reduced cross-sectional area conductively interconnects the other of said pair of terminal elements and the other of said pair of contacts. Said second fusible element includes a point of reduced cross-sectional area arranged inside said sub-housing and having a larger cross-sectional area than said predetermined minimum cross-sectional area. Fusible means normally preclude relative movement of said pair of contacts. Said fusible means include a solder joint arranged at the point of junction of said second fusible element and said other of said pair of contacts. Fuses embodying this invention further include a fusible shunt of-resistance wire shunted across said pair'of contacts, said point of reduced cross-sectional area of said second fusible element having a larger cross-sectional area than said predetermined minimum cross-sectional area and at least one point of said second fusible element having said predetermined minimum cross-sectional area.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal section of a fuse embodying the present invention taken substantially along ll of FIG. 2;

FIG. 2 is a longitudinal section of the same structure as shown in FIG. 1 taken substantially along lIll of FIG. I;

FIG. 3 shows a portion of FIG. 2 on a larger scale than FIG. 2;

FIG. 4 is a view of the structure shown in FIG. 3 seen in the direction of the arrow S of FIG. 3 and illustrates a step in the process of assembling the structure of FIGS. I-3; and

FIG. 5 is a time-current curve showing the process of interrupting an overload current.

DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION Referring now to the drawings, numeral I has been applied to indicate a tubular casing of electric insulating material, preferably a synthetic-resin-glass-cloth laminate. Casing 1 is closed by a pair of electroconductive terminal elements 2, preferably in the form of ferrules, or caps, mounted on the outer surface of easing 1. Casing 1 is filled with a pulverulent arc-quenching filler 3, preferably formed by a body of quartz sand. A pair of contacts 4,5 is arranged inside of easing I adjacent the center region thereof. Contact 4 is in the form of a metal container having a circular cross-section, and contact 5 is formed by a metal plunger arranged in part inside of container 4 and projecting with theright end to the outside of container 4. Helical spring 6 is arranged inside of container 4 and rests with one end thereof against flange 7 of plunger 5. Thus plunger 5 is spring-biased from right to left, as seen in FIGS. 1 to 3, i.e., biased to move into the inside of container 4. Such movement is normally prevented due to the presence of an annular solder joint 8 securing plunger 5 to container 4 in the position of parts 4,5 shown in FIG. 3. Upon softening of solder joint 8 plunger 5 may be moved from right to left under the action of spring 6.

Reference numeral 9 has been applied to indicate a sub-housing for contacts 4,5 arranged inside of easing I and separating contacts 4,5 from pulverulent arcquenching filler 3, and further forming a void 10 around plunger 5, i.e., a space free from filler 3. The left end of sub-housing 9 is tightly mounted on container 4, i.e., it is press-fitted on it. Sub-housing 9 is preferably formed by a length of cellulosic tubing which is an electric insulating material that can readily be squashed locally. As shown in FIGS. 2 and 3 the right end of tubular sub-housing 9 is squashed and the engaging wall ends of sub-housing 9 are fastened to each other by a fastener, e.g., staple 11. The squashed or flattened end of sub-housing 9 forms the void or cavity l0 enclosing the cone-shaped end of plunger 5 projecting out of container 4. The fuse includes a pair of fusible elements 12,13 in ribbon form which are preferably of sheet copper. Fusible element 12 will hereinafter be referred-to as first fusible element, and fusible element 13 will hereinafter be referred-to as second fusible element. First fusible element 12 has a plurality of serially related points of reduced cross-sectional area. These points are formed by circular perforations I4. Fusible element 12 is provided with four such perforations. It interconnects conductively the left terminal element or ferrule 2 and container 4. The right end of fusible element 12 is conductively connected to the bottom of container 4 by a joint 15 ofa solder having a relatively high softening, or melting, point. The crosssectional area of fusible element 12 is smallest at the points where perforations 14 are located or, in other words, perforations 14 form points of predetermined minimum cross-sectional area. The second fusible element 13 interconnects conductively the right terminal element or ferrule 2 and plunger 5. To this end plunger 5 is preferably provided with a groove into which the left end of fusible element 13 is inserted, and fusible element l3 and plunger 5 are conductively interconnected by a solder joint 16. While joint is formed by a solder having a relatively high softening point both joints 8 and 16 are formed by a solder having a relatively low softening point, or fusing point. Fusible element 13 is provided with three points 17,18 of reduced cross-sectional area. The point 17 of reduced crosssectional area is formed by a neck which is positioned inside of sub-housing 9 and forms a heater for solder joints 8 and 16. The two other points of reduced crosssectional area of fusible element 13 are formed bycircular perforations 18 which have the same diameter as perforations 14 of fusible element 12. The width and thickness of fusible elements 12,13 are equal. The cross-sectional area of neck 17 exceeds the crossseetional area of fusible elements 12 and 13 at points 14 and 18. The fastener or staple 11 projects transversely through the right flattened end of sub-housing 9 and also through the left end of fusible ribbon element 13.

Reference numeral 19 has been applied to indicate a shunt of a resistance wire, e.g., of nichrome. Shunt 19 is shunted across metal container 4, plunger 5, neck 17 and at least one of the points 18 of reduced crosssectional area fusible element 13 having the same minimum cross-sectional area as points 14 of fusible element 12. This mode of conductively interconnecting fusible elements 12,13 by shunt 19 is of critical importance since it results in performance characteristics which are significantly different from the performance characteristics of comparable prior art shunt arrangements. This willbe explained below in more detail.

The way in which the ends of resistance wire 19 are affixed to, and conductively connected to, fusible elements 12 and 13 is novel and may be applied to advantage in instances involving other overload current time lag interrupting means than parts 4,5,6,8 and 16. The ends 20 of shunt wire 19 are bent substantially circularly to form a pair of substantially circular loops. The

, inner diameter of these loops is substantially the same as the diameter of circular perforations 14 and 18. One of the wire loops 20 is arranged in coaxial relation with the perforation 14 immediately adjacent container 4. The otherof the wire loops 20 is arranged in coaxial relation with a perforation 18 of fusible element 13 spaced from plunger 5 by the interposition of neck 17 and the interposition of one of perforations 18 of fusible element 13. Reference numeral 21 has been applied to indicate a pair of fasteners e.g., rivets, but preferably eyelets having shanks and each projecting with the shank thereof through one of resistance wire loops 20 and through one of perforations 14,18 in fusible elements 12,13 and each clamping with a head or flange portion thereof one of loops 20 against one of fusible elements 12,13.

in the process of assembling the structure of FIGS. 1-3 the resistance wire 19 is initially positioned in the plane generally defined by fusible elements 12 and 13. Wire 19 has been shown in H0. 4 in solid lines in this initial position. Thereafter the portion of wire 19 intermediate the ends thereof is bent degrees out of the plane generally defined by fusible elements 12 and 13. This bending operation is necessary, ordesirable. in order to keep the spacing between the center portion of resistance wire 19 and easing 1 sufficiently large, i.e., as large as required.

Comparable prior art fuses, in particular fuses as disclosed in the aforementioned U.S. Pat. No. 3.483.501 perform fairly satisfactorily in d-c circuits if the overload current under interruption is relatively small, or in case of major fault currents. These fuses tend to fail completely in a wide intermediate current range, even in special versions thereof intended for d-c applications. I

To be more specific, prior art time-lag fuses for d-c applications may form a break at the point intended to form a break in case ,of small protracted overloads which break is capable of generating a sufficiently high are voltage to force the d-c current down to zero. In case of major fault currents series breaks are formed which may generate such a high arc-voltage that the fault current decays to zero before reaching its available level. In a wide range of intermediate overload currents a single break is formed in the current path of the fuse at a point thereof not coextensive with, but spaced from, the point of break capable of interrupting small overload currents. If such a single break is formed tively clearing overload currents in excess of those that can be cleared by the first interrupting mechanism, and smaller than those that bring into play the'second inter rupting mechanism. This will become more apparent from what follows below. I

1. Case of interruption of small overloads. In that instance solder joints 8 and 16 soften, allowing plunger '5 to move from right to left under the action of spring 6. As a result, an air break is formed between plunger 5 and fusible element 13. That air break is shunted by resistance wire 19 which fuses relatively rapidly and thus completes the interruption of the small overload .current.

2. Case of interruption of major fault currents. In that instance four series breaks are formed virtually simultaneously, forcing the current down to zero in currentlimiting fashion, i.e., at such a rapid rate that it never reaches its available level. Four series breaks have been sufficient to thus limit major fault currents in d-c circuits having a circuit voltage of 700 volts and a large time eonstant.'At the occurrence of major fault currents three series breaks are formed at the four'perforations 14 adjacent left ferrule 2, and a fourth series break is formed at the perforation 18 adjacent neck 17. A break may also be formed at neck 17 though its cross-sectional area exceeds that of the four aforementioned points 14 and18 of fusible elements 12,13. No breaks are formed at the perforations 14 and 18 of fusible elements 12 and 13 at the points thereof where fusible shunt 19 is affixed to them since the fusing i 't at these points is much larger than the fusing i at the aforementioned break-forming points.

3. Case of interruption of intermediate currents. Such currents do not cause fusion of solder joints 8 and 16 and formation of a break by separation of plunger 5 from fusible element 13. However, such currents cause formation of a single air break in the current path of the fuse by melting or fusion of neck 17. The arcvoltage generated at this single break reduces the current flow and causes a transfer of a portion of the total current of the fuse to shunt 19.

P10. 5 shows how a d-c overload current I ofintermediate magnitude is interrupted by the structure of FIGS. 1-3. During the period of time t, there is a small current decay due to the formation of an air break resulting from fusion of' neck 17. During the period t the current decays rapidly, primarily on account of the large positive temperature coefficient of resistance of a resistance wire of a metal such as nichrome. The rate of current decay at the beginning of the heating period of shunt l9 exceeds the rate of current decay at the end of the heating period. The current decay at the end of the heating period t is relatively small because the circuit under interruption supplies to wire 19 the heat of fusion thereof. At the end of the heating period t the overload current drops almost instantly to zero. The arc kindled across the gap formed by fusion and vaporization of neck 17 extinguishes shortly after shunt wire 19 begins to carry current.

Tests conducted with fuses embodying this invention as shown in F108. 1-3, inclusive, effectively interrupted d-c currents up to 6-7 times the minimum fusing currents by operation of plunger 5 and fusion of shunt l9, d-c currents over times the minimum fusing current were effectively interrupted by formation of four series breaks at three perforations l4 and one perforation 18, and d-c currents in the range from 6-7 to 10 times the minimum fusing current were effectively in terrupted by formation of an air break inside subhousing 9 by fusion and vaporization of neck 17 and subsequent fusion of fusible resistance wire element 19.

As mentioned above the cross-sectional area of neck 17 exceeds the cross-sectional area of the points of restricted cross-sectional area formed by the circular perforations in the fusible ribbons 12,13. l-lence neck 17 forms a break in series with the breaks formed at perforations 14 and 18 if major fault currents are of very high magnitude. It has been found essential for effective interruption of major fault currents that at least one point of minimum cross-sectional area immersed in pulverulent arc-quenching filler 3 be provided between neck 17 and the point at which shunt 19 is conductively connected to fusible element 13. Shunt 19 ought not to extend across all the points of reduced cross-sectional area of fusible element 12, and should preferably merely extend to the point of reduced cross-sectional area immediately adjacent to container 4.

I claim as my invention:

1. An electric fuse including in combination a. a tubular casing of electric insulating material;

b. a pair of electroconductive terminal elements closing the ends of said casing;

c. a pair of spring-biased relatively movable contacts arranged inside said casing adjacent the center region thereof;

d. a pulverulent arc-quenching filler inside said case. a sub-housing for said pair of contacts arranged inside of said casing separating said pair of contacts from said pulverulent arc-quenching filler;

f. a first fusible element in ribbon form having a plurality of serially related points of'reduced crosssectional area, said plurality of points of reduced cross-sectional area having a predetermined minimum cross-sectional area and said first fusible element conductively interconnecting one of said pair of terminal elements and one of said pair of contacts;

g. a second fusible element in ribbon form having a plurality of serially related points of reduced crosssectional area, said second fusible element conductively interconnecting the other of said pair of terminal elements and the other of said pair of contacts, and said second fusible element including a point of reduced cross-sectional area arranged inside said sub-housing and having a larger crosssectional area than said predetermined minimum cross-sectional area;

h. fusible means normally precluding relative movement of said pair of contacts, said fusible means including a solder joint arranged at the point ofjunction of said second fusible element and said other of said pair of contacts; and

i. a fusible shunt of resistance wire shunted across said pair of contacts, said point of reduced crosssectional area of said second fusible element having a larger cross-sectional area than said predetermined minimum cross-sectional area and at least one point of said second fusible element having said predetermined minimum cross-sectional area.

2. An electric fuse as specified in claim 1 wherein said points of reduced cross-sectional area having a minimum cross-sectional area are formed by circular perforationsin said first fusible element and in said second fusible element, wherein said shunt of resistance wire is circularly bent at the ends thereof to form a pair of circular loops having substantially the same diameter as said circular perforations, and wherein said circular loops of said resistance wire are clamped against said first fusible element and against said second fusible element by fasteners projecting through said circular loops and projecting through said circular perforations.

3. An electric fuse as specified in claim 2 wherein said fasteners are formed by eyelets.

4. An electric fuse as specified in claim 2 wherein the portion of said shunt resistance wire situated between the ends thereof is bent into a plane substantially at right angles to the plane generally defined by said first fusible element and by said second fusible element.

5. An electric fuse including in combination a. a tubular casing of a synthetic-resin-glass-cloth laminate;

b. a pair of ferrules closing the ends of said casing;

c. a body of quartz sand inside said casing;

d. a metal plunger arranged in part inside of a metal container and'projecting with one end thereof to the outside of said container, said plunger and said container being arranged inside said casing adjacent the center region thereof;

e. a helical spring inside said container biasing said plunger for movement into the inside of said container;

f. a sub-housing inside said casing separating said plunger and said container from said body of quartz sand, said sub-housing having one end tightly mounted on said container and having a flattened end enclosing said end of said plunger projecting outside said container;

g. a first fusible element in ribbon form having a plurality of serially related points of reduced crosssectional area each having a predetermined minimum cross-sectional area, said first fusible element conductively interconnecting one of said pair of ferrules and said container;

h. a second fusible element in ribbon form having a plurality of serially related points of reduced crosssectional area and conductively interconnecting the other of said pair of ferrules and said end of said plunger projecting outside said container, said second fusible element including a neck arranged inside said subhousing and having a larger crosssectional area than said predetermined minimum cross-sectional area;

i. solder means normally precluding relative movement of said plunger and said metal container, said solder means including a solder joint conductively interconnecting said plunger and said second fusible element; and

j. a shunt of resistance wire shunted across said container, said plunger, said neck of said second fusible element and at least one point of reduced crosssectional area of said second fusible element having said predetermined minimum cross-sectional area.

6. An electric fuse as specified in claim wherein said shunt of resistance wire does not extent across all of said points of reduced cross-sectional area of said first fusible element.

7. An electric fuse as specified in claim 5 wherein said shunt of resistance wire extends merely to the point of reduced cross-sectional area of said first fusible element immediately adjacent said container.

8. An electric fuse including in combination a. a tubular casing of electric insulating material;

b. a pair of electroconductive terminal elements closing the ends of said casing;

c. a pulverulent arc-quenching filler inside said casd. fusible means conductively interconnecting said terminal elements, said fusible means including fusible element means in ribbon form having a plurality of serially related circular perforations and said fusible means further including low fusing point overload current interrupting means arranged adjacent the center region of said casing;

e. a shunt in form of a wire extending across said low fusingvpoint overload current interrupting means, the ends of said wire being bent substantially circularly to form a pair of loops each having substantially the same diameter as one of said plurality of circular perforations of said fusible element means in ribbon form and each being arranged in coaxial relation to one of said circular perforations of said fusible element means in ribbon form; and

f. a pair of fasteners having shanks and each projecting with the shank thereof through one of said pair of loops of said shunt in form of a wire, and each projecting with the shank thereof through one of said plurality of circular perforations of said fusible element means in ribbon form and each clamping one of said pair of loops against said fusible element means in ribbon form. 

1. An electric fuse including in combination a. a tubular casing of electric insulating material; b. a pair of electroconductive terminal elements closing the ends of said casing; c. a pair of spring-biased relatively movable contacts arranged inside said casing adjacent the center region thereof; d. a pulverulent arc-quenching filler inside said casing; e. a sub-housing for said pair of contacts arranged inside of said casing separating said pair of contacts from said pulverulent arc-quenching filler; f. a first fusible element in ribbon form having a plurality of serially related points of reduced cross-sectional area, said plurality of points of reduced cross-sectional area having a predetermined minimum cross-sectional area and said first fusible element conductively interconnecting one of said pair of terminal elements and one of said pair of contacts; g. a second fusible element in ribbon form having a plurality of serially related points of reduced cross-sectional area, said second fusible element conductively interconnecting the other of said pair of terminal elements and the other of said pair of contacts, and said second fusible element including a point of reduced cross-sectional area arranged inside said sub-housing and having a larger cross-sectional area than said predetermined minimum cross-sectional area; h. fusible means normally precluding relative movement of said pair of contacts, said fusible means including a solder joint arranged at the point of junction of said second fusible element and said other of said pair of contacts; and i. a fusible shunt of resistance wire shunted across said pair of contacts, said point of reduced cross-sectional area of said second fusible element having a larger cross-sectional area than said predetermined minimum cross-sectional area anD at least one point of said second fusible element having said predetermined minimum cross-sectional area.
 2. An electric fuse as specified in claim 1 wherein said points of reduced cross-sectional area having a minimum cross-sectional area are formed by circular perforations in said first fusible element and in said second fusible element, wherein said shunt of resistance wire is circularly bent at the ends thereof to form a pair of circular loops having substantially the same diameter as said circular perforations, and wherein said circular loops of said resistance wire are clamped against said first fusible element and against said second fusible element by fasteners projecting through said circular loops and projecting through said circular perforations.
 3. An electric fuse as specified in claim 2 wherein said fasteners are formed by eyelets.
 4. An electric fuse as specified in claim 2 wherein the portion of said shunt resistance wire situated between the ends thereof is bent into a plane substantially at right angles to the plane generally defined by said first fusible element and by said second fusible element.
 5. An electric fuse including in combination a. a tubular casing of a synthetic-resin-glass-cloth laminate; b. a pair of ferrules closing the ends of said casing; c. a body of quartz sand inside said casing; d. a metal plunger arranged in part inside of a metal container and projecting with one end thereof to the outside of said container, said plunger and said container being arranged inside said casing adjacent the center region thereof; e. a helical spring inside said container biasing said plunger for movement into the inside of said container; f. a sub-housing inside said casing separating said plunger and said container from said body of quartz sand, said sub-housing having one end tightly mounted on said container and having a flattened end enclosing said end of said plunger projecting outside said container; g. a first fusible element in ribbon form having a plurality of serially related points of reduced cross-sectional area each having a predetermined minimum cross-sectional area, said first fusible element conductively interconnecting one of said pair of ferrules and said container; h. a second fusible element in ribbon form having a plurality of serially related points of reduced cross-sectional area and conductively interconnecting the other of said pair of ferrules and said end of said plunger projecting outside said container, said second fusible element including a neck arranged inside said subhousing and having a larger cross-sectional area than said predetermined minimum cross-sectional area; i. solder means normally precluding relative movement of said plunger and said metal container, said solder means including a solder joint conductively interconnecting said plunger and said second fusible element; and j. a shunt of resistance wire shunted across said container, said plunger, said neck of said second fusible element and at least one point of reduced cross-sectional area of said second fusible element having said predetermined minimum cross-sectional area.
 6. An electric fuse as specified in claim 5 wherein said shunt of resistance wire does not extent across all of said points of reduced cross-sectional area of said first fusible element.
 7. An electric fuse as specified in claim 5 wherein said shunt of resistance wire extends merely to the point of reduced cross-sectional area of said first fusible element immediately adjacent said container.
 8. An electric fuse including in combination a. a tubular casing of electric insulating material; b. a pair of electroconductive terminal elements closing the ends of said casing; c. a pulverulent arc-quenching filler inside said casing; d. fusible means conductively interconnecting said terminal elements, said fusible means including fusible element means in ribbon form having a plurality of serially related circular perforations and said fusible means further including low fusing point overload current interrupting means arranged adjacent the center region of said casing; e. a shunt in form of a wire extending across said low fusing point overload current interrupting means, the ends of said wire being bent substantially circularly to form a pair of loops each having substantially the same diameter as one of said plurality of circular perforations of said fusible element means in ribbon form and each being arranged in coaxial relation to one of said circular perforations of said fusible element means in ribbon form; and f. a pair of fasteners having shanks and each projecting with the shank thereof through one of said pair of loops of said shunt in form of a wire, and each projecting with the shank thereof through one of said plurality of circular perforations of said fusible element means in ribbon form and each clamping one of said pair of loops against said fusible element means in ribbon form. 